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Fperotti's Avatar
 
Join Date: Nov 2004
Location: Buenos Aires, Argentina
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Porsche Crest 2.4T MFI rebuild using 2.2 S pistons.

Hi all,

I'll rebuild my 2.4T MFI engine. My idea is to hone the cylinders -the wear is evident- to std. + 0.25mm. Then, I'd like to use 2.2S pistons. i heard this is a nice conversion: more power and more torque (what I love). Hear begins my doubts. Do you know wich changes should I perform in my engine to do this succesfully? For instance, in what way should I recalibrate the injection pump? Should I change the camshafts?. The valve to piston clearance is ok in this configuration?

I guess I can have the forged piston set supplied in my country. I guess if I use forged pistons the gap between piston and cylinder will be different than the one specified for cast-irons cylinders. Does anybody knows where can I find these values and what kind of rings -besides 0.25mm bigger bore consider- should I use?

Thank you very much in advance.
Fernando.
Old 01-28-2005, 08:29 PM
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If you're going to use 2.2S pistons, you can't bore it. If you are going to use custom new pistons, bore the cylinders to 85mm, not 84.25. The extra .75mm will bring you to an "actual" 2.4--2397cc. You can of course change the cams to E, S? With S pistons or custom higher compression pistons the valve clearance most likely won't be a problem. The MFI pump will be ok.

Peace
Old 01-28-2005, 11:41 PM
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You're going to wind up with a lot of CR and a mild cam. The dynamic CR (the CR based on when the intake valve closes) will be pretty high. You might need to back off on the ignition timing (and/or re-curve the distributor) to prevent detonation at peak advance. Changing to a more radical cam (let's a an E) will reduce the dynamic CR since the intake valve stays open longer, and will help your engine to rev a little higher in the rev range.
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Old 01-29-2005, 04:40 AM
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Thanks ytnuklr and John,

Yes, I'll use custom pistons, not for performance but for economy. In that way I won't have to buy new cylinders. Really, if it is possible I'd prefer not to change the cams in order to keep the MFI pump as it is. I don't know how complex could be to reset to S specification the pump. I guess I should change the pump cam, am I correct?. Maybe using E shape pistons will be smarter, what do you think?

In Wayne's book the modificaction we are talking about is mentionated and recommended but I could not find additional information about it. Do you know where can I find more information regarding the 2.2S pistons using in 2.4T motors?

Thanks,
Fernando.
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Old 01-29-2005, 06:53 AM
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